Vegetable materials often consist of high valued materials such as protein and valued non-structural carbohydrates in combination with low valued fibrous structures such as hulls and straw. Some, but not all, of the valuable proteins and carbohydrates are water soluble. Many of the non-water-soluble valued proteins and non-structural carbohydrates are present as particles smaller than the fiber.
One particular group of products which contain useful amounts of carbohydrate and protein are oilseed materials from which oil has been extracted. In cases where they have been de-fatted by a cold press process to remove oil, they also contain a considerable amount of residual oil. Less oil is present when they are de-fatted by a solvent process. In particular, the product remaining after oil has been removed from canola (rapeseed), which is called defatted canola flake, is a rich source of valued proteins and carbohydrates. These oilseed materials also contain low valued fibrous materials such as hulls and straw, which should be removed to give a higher-value product.
One way to separate the low valued fibrous material from water soluble valuable material is by aqueous extraction. In aqueous extraction processes, water is added to the vegetable starting material to form a thick slurry. Typically, the slurry is composed of three distinct phases: a liquid containing the soluble components of the starting material, a light solids phase composed of fine particles in suspension; and a heavy solids phase composed of fibrous materials such as hulls and residual straw. Extraction processes are frequently designed to remove only the liquid phase from the slurry. Such processes utilize established separation techniques such as centrifugation that are designed to generate a clear liquid extract. However, the fine particles in suspension, which contain valued proteins and carbohydrates, are directed toward the solid residue in any separation system that generates a clear extract. Chemical modification is frequently employed to improve the solubility of proteins in the slurry. However, these modifications add cost to the process and can damage the nutritional value of the extract.
It would be advantageous to develop a mechanical separation system that directs the fines toward the extract and in so doing generates an extract consisting of both the liquid phase and the light solids phase containing the valued fines. This would permit recovery of both soluble and non-soluble (small particulate) non-fibrous non-structural material in the extract. The non-soluble, small particulate material is frequently high in valuable materials such as carbohydrates and proteins.
However, when many crushed vegetable products are slurried, the liquid portion of the slurry is thick and viscous. This is because of various water-soluble or partly soluble proteins and carbohydrates which pass into the water phase of the slurry. In defatted oilseeds, some residual oil may also be present. Also, after the defatting process (particularly in the case of making defatted canola flake), some small particles of cell meat may be present. These are high in protein, and are therefore especially valuable to recover.
In the case of a thick viscous starting slurry of vegetable material, such as a slurry of oil-extracted canola flakes, obtaining efficient extraction of liquid plus light phase solids has certain difficulties. Compression-based filtration can be used to obtain an extract containing light phase solids. In this process the slurry is pressed with a filter media having pore openings or aperture sizes that permit passage of the light solid phase in the slurry, while retaining the heavy solids as an extracted presscake. An example of this type of process is shown in PCT Published Application 03/043438 of MCN BioProducts Inc. However, the thick viscous nature of the slurry results in compacting of the filter media, poor separation per unit area of filter media and considerable extrusion of slurry from the sides of the filter media. Therefore, direct processing of a viscous slurry of vegetable material by compression filtration as described in the PCT published application requires extensive filtration area and slow process rates. Substantial equipment is required, which drives up the cost of production.
Willis et al U.S. Pat. No. 5,814,230 describes a process and apparatus for the separation of coarse and ultrafine solids from a liquid stream. In this process multiple filter screens of various pore opening sizes are passed repeatedly through the solids containing feed suspension until a filter cake builds up on the surface of the screens and a clarified solids-free liquid phase is generated. The solids are subsequently discharged from the screen and dewatered by means such as vibration and direct blasts of air or by compression-based dewatering. The multiple screens of progressively smaller pore openings are designed to generate a solids-free clarified extract and thus in the case of extraction of a vegetable slurry the valued fragments of cell meats would not reside in the extract generated by this process.
Glorer U.S. Pat. No. 4,975,183 teaches that a stirring apparatus can be automatically raised and lowered during pressure driven filtration of a solids-containing slurry to create an even distribution of cake on the filtering surface and thus improved performance of the filtration process. This process can be described as an improvement of conventional single stage pressure-driven filtration.
Lindoerfer et. al U.S. Pat. No. 4,921,615 teach a multi-stage pressure-driven method for solids removal from viscous liquids. In this process, the viscous feed slurry containing solids is pressure filtered in a series of steps involving filter material of progressively decreasing pore size. This process is designed to generate a clear liquid extract.
Impeller-driven filtration is a known filtration technique. In such filtration, a rotating impeller blade is passed close to a filter medium, as a slurry is passed over the same filter medium. The action of the impeller repeatedly sweeps the slurry over the filter medium, and minimizes compacting of the slurry on the filter media. However, impeller-driven filtration tends to leave a residue which remains high in water content.
Centrifugation is a known filtration technique. However, centrifugation is not effective with viscous vegetable slurries, because the viscous nature of the slurry does not permit adequate separation using established centrifugal filtration processes.
Thus, current separation apparatus and processes do not provide a practical and cost-effective means for separating water-soluble proteins and small particles of cell meats (when present) from the remaining vegetable matter, particularly when a viscous slurry is involved. Additionally, they tend to leave the residue fairly wet, so that a considerable amount of energy is required for drying.